The present invention relates to a microcomputer having a high-speed operation mode and a low-speed operation mode, and particularly to a microcomputer having a high-speed sense amplifier for reading instructions out of an instruction memory.
In general, a high frequency oscillator is used to apply a high frequency clock to a microcomputer in the high-speed operation mode, while a low frequency oscillator is used in the low-speed operation mode. In order to increase the operating speed of a microcomputer, both the instruction execution speed and the instruction reading speed must be increased. To increase the instruction reading speed, a high-speed sense amplifier, such as a current mirro type sense amplifier, will be required as a memory reading circuit. Since a sense amplifier of this type requires a large drive current, a large amount of power is consumed.
In the high-speed operation mode, the instruction reading cycle becomes short because the instruction execution speed is increased and because the high frequency clock is used. However, the instruction reading period can not be shortened to less than a period determined by the characteristics of the sense amplifier and the memory, to insure that an instruction fetch into the central processing unit (CPU) can be carried out. Thus, it will be required in the high-speed operation mode to use a sense amplifier which is activated either full time or with a large duty rate.
On the other hand, when the low frequency oscillator is connected to the microcomputer in the low-speed operation mode, low power consumption is required. In this case, a low frequency clock is applied to the microcomputer. However, a large amount of power is still consumed b the above mentioned sense amplifier because it is actvated at the same duty rate as that in the high-speed opration mode. To avoid power consumption by this sense amplifier, a low-power sense amplifier is required. However, since a microcomputer having such a low-power sense amplifier has not been developed, a long development term and design change are necessary.